This invention relates generally to drilling. More specifically the invention relates to drilling in earthen formations.
When drilling in hard mediums, for example earthen formations, a common problem is rotational vibration caused by a “stick-slip” condition. Stick-slip occurs when the drill bit either “bites” too much or too little of the medium to cause further boring into the medium. When the drill bit bites too much of the medium, each of the cutters on the drill bit is engaging too much medium without enough torque to cause the medium to either be sheared or otherwise removed from the cavity. When the drill bit bites too little of the medium, the path of least resistance for the drill bits may be to skip over the medium rather than remove material from the cavity. Both stick and slip occurrences will cause torsional vibrations in the drillstring connecting the drill bit to the drill's rotational power source.
Torsional vibrations can be so severe as to stop the bit from moving forward, for example, when a backward vibration is equal to the forward rate of rotation. Once this occurs, the difference between the static and dynamic friction characteristics of the drill bit on the medium can make the oscillations self sustaining. Torsional vibrations caused by stick-slip can also reduce the speed at which a cavity is drilled into the medium. Furthermore, such torsional vibrations can reduce the life of the drill bit and associated drilling equipment, such as for example the rotational power source.
While the properties of the medium may at least partially affect the likelihood of stick-slip occurring, the properties of the medium may not be controllable, especially in earthen drilling operations. However, the amount of torque may also at least partially affect the likelihood of stick-slip occurring, and may be controlled.